The present invention relates to a photochemical process and more particularly to a photochemical process for altering the isotopic composition of mercury.
The excitation of specific mercury isotopes by photochemical means is well known in the art. For example, the paper by Webster and Zare, "Photochemical Isotope Separation of .sup.196 Hg by Reaction with Hydrogen Halides" J. Phys. Chem. 85, 1302 (1981) discloses such excitation. Mercury vapor lamps are commonly used as an excitation source of mercury isotopes for specific photochemical reactions. To be successful, photochemical separation of a single isotope requires that the spectral band width of the exciting mercury radiation must be sufficiently narrow to excite only the isotope of interest. The specificity depends upon the spectral band width of the source. The rate and extent of separation of the particular isotope from the feed stock can be strongly dependent on the intensity of the radiation emitted from the mercury source.
A weakly ionized plasma of mercury and rare gases under low pressure, in the order of 1 to 3 torr, forms the basis of the fluorescent lamp. Electrical energy is converted to natural mercury resonance radiation at 253.7 nm. at an efficiency of 55 to 65%. This radiation, in fluorescent lamps, is converted to visible light by solid phosphors that are coated upon the lamp envelope. The efficiency of the 253.7 nm. resonance radiation emitted from excited mercury atoms in the plasma is reduced because the resonance radiation is absorbed and reemitted many times by ground state mercury atoms during its escape to the walls of the discharge tube. This trapping of resonance radiation prolongs the effective lifetime of the excited atoms and increases the opportunity for radiationless energy conversion which reduces efficiency.
It is known that the 253.7 nm. resonance line of mercury is composed of 5 hyperfine components, principally the result of isotope shifting. As is known, the .sup.196 Hg isotope in natural mercury does not contribute substantially to the radiation because of its low concentration, nor does its emission and absorption heavily overlap with the other hyperfine components. Therefore, by increasing its concentration, an additional channel for the 253.7 nm. photons is provided which reduces the average imprisonment time and increases radiation efficiency.
Devices have previously been disclosed to enrich the .sup.196 Hg in mercury feed stocks. In the paper of McDowell et al., "Photochemical Separation of Mercury Isotopes" Can. J. Chem., Vol. 37, 1432 (1959), a disclosure is made of reacting .sup.202 Hg(6.sup.3 P.sub.1) atoms that are contained in natural mercury with hydrogen chloride with a photochemical reaction in which the .sup.202 Hg atoms are excited during the reaction to precipitate an .sup.202 Hg.sub.2 Cl.sub.2. EQU .sup.202 Hg(6.sup.3 P.sub.1)+HCl.fwdarw..sup.202 HgCl+H. (1)
As described in a paper by Mark Grossman and Jakob Maya delivered at the International Quantum Electronics Conference, June 1984, very high enrichment of .sup.196 Hg can be achieved in a photochemical reaction using a natural mercury vapor filter. When radiation from a microwave lamp containing mercury enriched to 35% in .sup.196 Hg is used in a filter, the filter eliminates substantially all of the .sup.non-196 Hg component radiation permitting an isotopically selective primary excitation of the .sup.196 Hg isotope. Selective excitation of .sup.196 Hg(6.sup.3 P.sub.1) in natural mercury vapor is obtained by an RF-excited, Hg and rare gas source whose emission is filtered through an atomic vapor filter before it enters into the reaction zone.
The photochemical enrichment of .sup.196 Hg occurs via the reaction: EQU .sup.196 Hg(6.sup.3 P.sub.1)+HCl.fwdarw..sup.196 HgCl+H (2)
In the above-referenced papers of Webster et al. and Grossman et al., butadiene (C.sub.4 H.sub.6) was used with HCl as a carrier medium for the feedstock Hg in order to mix with and transport the Hg into the photochemical reaction zone. The use of the butadiene component is due to the following reactions that accompany (1) and (2), above. EQU .sup.196 Hg(6.sup.3 P.sub.1)+HCl.fwdarw..sup.196 Hg(6.sup.1 S.sub.O)+H+Cl (3) EQU H+HCl.fwdarw.H.sub.2 +Cl (4)
The free Cl radical reacts in a non-isotope specific way to form .sup.N HgCl, i.e. EQU .sup.N Hg(6.sup.1 S.sub.O)+Cl.fwdarw..sup.N.sup.M HgCl (5)
where M designates a third body or surface, and N represents any Hg isotope. Comparing Equations (2) and (5), one sees that the reaction product isotope specificity is "scrambled" due to reaction (5). Butadiene was thought to significantly reduce the rate of Equation (5) and therefore increase the product enrichment. This rate reduction of Equation (5) is thought to occur via a polymerizing reaction in which the H and/or Cl are attached to the C.sub.4 H.sub.6 via saturation of the multiple bonds in C.sub.4 H.sub.6, thereby reducing the rate of (5).
While high enrichments can be achieved using C.sub.4 H.sub.6, three major problems exist with the use of C.sub.4 H.sub.6. First, C.sub.4 H.sub.6 has a relatively high cost. Second, polymerized C.sub.4 H.sub.6 compounds are difficult to remove from the effluent which further prevents maintaining a relatively low contamination level in the flow system. Lastly, the quenching rate of .sup.196 Hg(6.sup.3 P.sub.1) by butadiene is relatively high.
The relatively high quenching rate may explain the low utilization factor achievable with the use of butadiene. A carrier gas which would increase the utilization factor and which reduces the contamination of the effluent would be desirable.